A constant voltage battery charger is a basic charger in which battery voltage is monitored during the charging sequence and once a predetermined battery voltage is reached charging stops. Basically, the constant voltage charger operates by rapid charging a battery at a predetermined charge current and then decreasing the charge current to a minimum charge, commonly referred to as trickle charge current, once the predetermined battery voltage is reached. A problem can arise, however, as the battery starts to heat up as it approaches its end of charge. The rise in battery temperature can cause a slight slump or dip in the battery voltage, and even though the battery is essentially fully charged, the constant voltage charger senses this dip in battery voltage as an incompletely charged battery. Rather than reverting to trickle charge, the constant voltage charger may continue to rapid charge a battery which is essentially fully charged. A self-perpetuating cycle of overcharging can occur where the current rises, causing the battery to heat, which in turn causes the voltage to drop, which further causes the current to increase. FIG. 1 shows a prior art graph depicting an example of this overcharging phenomena. Graph 100 shows curves comparing battery temperature 102, battery terminal voltage 104, and battery charge current 106 versus time for a battery being charged at its rated charge current. As seen on the graph 100, once a battery approaches its fully charged state (designator 108), a slight rise in battery temperature 102 causes the battery voltage 104 to drop slightly, causing the charge current 106 to ramp up (as seen between designators 108, 110), thereby increasing the charge current on an already fully charged battery.
This overcharging phenomena can also occur when the battery temperature rises as a result of a rise in ambient temperature, such as in an automotive environment. The voltage of a fully charged battery will vary depending on the ambient temperature. FIG. 2 shows a graph of typical battery charge curves for a 7.5 volt nickel cadmium battery charged at a rapid charge rate of 1 C (current=rated capacity of battery) in three different ambient temperature environments. Curve 202 represents the battery being charged at -30 degrees Celsius (.degree.C.). Curve 204 represents the battery being charged at room temperature, and curve 206 represents the battery being charged at +60.degree. C. As seen from the graph 200, the peak battery voltage varies from approximately 10.5 volts at the colder temperature to approximately 9.5 volts at the hotter temperature. Thus, if the battery cutoff voltage in the charging sequence is not set low enough, then an overcharging condition can occur at hotter temperatures as the charger continually attempts to rapid charge a battery which will never achieve the preset cutoff voltage. Unfortunately, setting the cutoff voltage to a lower voltage setting has the disadvantage of being unable to fully charge the battery to its full capacity when this same battery is being charged at room temperature or colder temperatures.
Accordingly, there is a need for an improved charging technique which addresses the problems described above.